Add ycm formula

[traversaro]

ycm (https://github.com/robotology/ycm) is used for several robotlogy superbuild and projects.
It is typically bootstrapped by the superbuilds, but this approach has some problems (see for example [1]). It would be convenient if we had an homebrew formula for ycm so macOs user could grab ycm directly from homebrew, avoiding the need for bootstrapping.

[1] robotology/ycm-cmake-modules#105

[francesco-romano]

done in 352aed0